(1) Field of the Invention
The present invention relates to methods for forming porous layers on the outer surfaces of electrode hoops of nonaqueous electrolyte secondary batteries or other batteries.
(2) Description of Related Art
As portable electronic devices and cordless electronic devices have come to be more widely used, expectations are growing for lightweight nonaqueous electrolyte secondary batteries with high energy density. However, an active material of such a nonaqueous electrolyte secondary battery is made of highly reactive lithium, and therefore a short circuit between a positive electrode and a negative electrode under abnormal conditions generates heat. Due to the generated heat, a resinoid microporous membrane separator for isolating the positive electrode from the negative electrode melts around the area in which the electrodes are short-circuited, thereby increasing the area in which the electrodes are short-circuited and thus generating abnormal superheat. In order to prevent this, a technique is being used in which, in case of the occurrence of a short circuit between electrodes, superheat due to an increase in the area where the electrodes are short-circuited is suppressed by combining a porous refractory layer having a refractory resin or an inorganic oxide as the main ingredient with the resinoid microporous membrane separator.
Such a porous refractory layer is formed on the outer surface of a positive electrode or a negative electrode (hereinafter, referred to as an “electrode”) without loss of the design capacity of a battery to have a thickness of 2 through 10 μm. A method in which a coating fluid serving as a precursor of a porous refractory layer is transferred to a gravure roll provided with a plurality of grooves and the transferred coating fluid is applied to the outer surface of a target electrode hoop (hereinafter, referred to as “gravure method”) is preferably employed as a method for forming a layer of a thickness as described above with high accuracy. In order to realize the gravure method, the direction in which an electrode hoop travels is allowed to become opposite to the direction of rotation of a gravure roll. In this way, a thin coating membrane can be formed on the outer surface of an object to be coated with a coating fluid with high accuracy.
By the way, from the viewpoint of improving the productivity of electrodes, electrodes each serving as a base of a porous refractory layer are typically processed in the following manner: A core that is several times as wide as each electrode itself is prepared; a plurality of linear mixture layers containing an active material are formed on the core; and then the core is cut into pieces each having one of the linear portions of the mixture layer. In view of the above, a plurality of porous refractory layers need be formed in consideration of the shape of each electrode.
A method in which a masking tape is bonded to the outer surface of a core, then a mixture layer is formed on the masking tape, and thereafter the masking tape is removed has been disclosed, as a method for forming a mixture layer on the outer surface of a core with high accuracy, in Japanese Unexamined Patent Publications Nos. 2005-183181, 2005-216722 and 2005-216723. Application of this method allows a plurality of linear porous refractory layers each having a predetermined width to be formed on the outer surface of an electrode.
By the way, in a case where an electrode hoop in which a plurality of linear mixture layers containing an active material are placed on a core is to be formed on the precondition that the core is partially exposed at both lateral end parts of the electrode hoop, any one of the following methods will be employed.
(1) A mixture paste (a precursor of mixture layers) is applied to a core, and then part of this mixture paste is scraped off the core before drying of the mixture paste such that a plurality of linear mixture layers are left on the core.
(2) A mixture paste is applied to a core, and then a mixture layer formed by drying the mixture paste is partially scraped off the core such that a plurality of linear mixture layers are left on the core.
(3) A plurality of linear mixture pastes are applied onto a core at fixed intervals, thereby forming a plurality of linear mixture layers.
For the electrode hoop formed by each of the above-mentioned methods, the location and width of each of a plurality of linear mixture layers (hereinafter, referred to “the lateral location of each mixture layer”) frequently vary due to the following reasons. More particularly, when the electrode hoop is formed by each of the methods (1) and (2), the locations of both lateral ends of the initially applied mixture paste vary. The reason for this is that the amount of the mixture paste spread toward two of exposed parts of the core corresponding to both lateral end parts thereof varies according to variations in the properties of the mixture paste with time. When the mixture paste or the mixture layer obtained by drying the mixture paste is divided into a plurality of portions in a later process, the both lateral ends of the initially formed mixture paste will become part of the outermost ones of the plurality of linear mixture layers. The width of each of the outermost ones of the plurality of linear mixture layers varies independently of the other ones of the plurality of linear mixture layers. Furthermore, when the electrode hoop is formed by the method (3), variations in the properties of the mixture paste with time make it difficult to keep the properties of the applied linear mixture pastes fixed. Therefore, the width of each of the linear mixture layers varies independently of the other linear mixture layers.
Since the lateral location of each mixture layer varies independently of the other mixture layers, the location at which a porous refractory layer to be formed on the mixture layer is formed must also be changed according to variations in the lateral location of the mixture layer.
However, in a case where a plurality of linear porous refractory layers are formed using the above-described masking tape to each have a predetermined width, this causes the following problems. More particularly, the location of a part of a core to which the masking tape is bonded needs to be frequently changed according to variations in the lateral location of each of mixture layers. When the location of the part of the core to which the masking tape is bonded is changed, the softness of the masking tape may cause the masking tape to become crinkled. In this case, an effect arising from the masking tape (hereinafter, referred to as “masking effect”) cannot be sufficiently provided.
On condition that when an electrode hoop is cut into pieces, a porous refractory layer is present, due to an insufficient masking effect, at a location at which a core is cut and at which the porous refractory layer should be essentially absent, burrs of the porous refractory layer may be produced at a location at which the core is cut. When such burrs are mixed into final products, i.e., batteries, this causes short circuits inside the batteries, leading to a decrease in the reliability of the products.